As a first step, we want to have just an ability to have multiple shadow root. In this step, we don't consider how elements having multiple shadow roots are rendered.
Note that some kinds of elements have shadow tree by own. We should decline to add shadow root in such elements.

(In reply to comment #1)
> Note that some kinds of elements have shadow tree by own. We should decline to add shadow root in such elements.
Why? A large reason of introducing multiple shadow subtrees per element is to _allow_ overriding shadow DOM of an element with a "built-in" shadow subtree. Can you explain what pitfalls I am not seeing here?

(In reply to comment #2)
> (In reply to comment #1)
> > Note that some kinds of elements have shadow tree by own. We should decline to add shadow root in such elements.
>
> Why? A large reason of introducing multiple shadow subtrees per element is to _allow_ overriding shadow DOM of an element with a "built-in" shadow subtree. Can you explain what pitfalls I am not seeing here?
Ah... Sorry, I didn't understand the motivation of multiple shadow subtrees. I was afraid that a user breaks current input implementation...
OK. That being the case, I don't have any reason to prevent overriding "built-in" shadow subtree.